Hello,
how can I fix this?

thanks!
What exact application are you starting when this occurs? The nRFConnect, or any of the apps included in nRFConnect? Please try uninstalling the application, then try deleting C:\Users\***\AppData\Roaming\nrfconnect\settings.json from your computer manually before reinstalling the application. You can also try opening this sources.json file to see what happens. Truth is I haven't seen this error message before.
Best regards,
Simon
What exact application are you starting when this occurs? The nRFConnect, or any of the apps included in nRFConnect? Please try uninstalling the application, then try deleting C:\Users\***\AppData\Roaming\nrfconnect\settings.json from your computer manually before reinstalling the application. You can also try opening this sources.json file to see what happens. Truth is I haven't seen this error message before.
Best regards,
Simon